Metal electrodes and coatings therefor

ABSTRACT

Conductive coating for metal electrodes, consisting essentially of a compound ABO4 with a structure of the rutile-type, where A represents an element in the trivalent state selected from the group consisting of Rh, Al, Ga, La, and the rare earths and B represents an element in the pentavalent state selected from the group consisting of Sb, Nb and Ta, and an oxide of the type MO2 where M is selected from the group consisting of Ru and Ir.

United States Patent [191 Degueldre et al.

METAL ELECTRODES AND COATINGS THEREFOR Inventors: Louis Degueldre;Charles Killens;

Louis Bourgeois, all of Brussels, Belgium Assignee: Solvay & Cie,Brussels, Belgium Filed: July 3, 1972 Appl. No.: 268,552

Foreign Application Priority Data July 9, 1971 Luxembourg 63506 US. Cl.204/290 F, 204/290 R, 204/291 Int. Cl B0lk 3/06 Field of Search204/290F, 290 R, 291

References Cited UNITED STATES PATENTS 1/1970 Bianchi et a1 204/242 Nov.19, 1974 3,562,008 2/1971 Martinsons 117/221 3,632,498 l/1972 Beer204/290 F 3,701,724 10/1972 Entwisle 204/290 F Primary Examiner-F. C.Edmundson Attorney, Agent, or Firm-Spencer & Kaye [5 7] ABSTRACT 5Claims, No Drawings METAL ELECTRODES AND COATINGS THEREFOR BACKGROUND OFTHE INVENTION The present invention relates to a new type of coatingintended for constituting the active surface of a metal electrode madeat least superficially of titanium or a titanium alloy and optionallycontaining a core of a better conducting material than titanium, forexample of copper, aluminum, iron or alloys of these metals.

Electrodes provided with a coating having catalytic properties accordingto the present invention may be used in various electrochemicalprocesses such as cathodic protection, desalination or purification ofwater, electrolysis of water or hydrochloric acid, production of currentin a fuel cell, reduction or oxidation of organic compounds of theelectrolytic manufacture of per salts, but they are particularly usefulas anodes in the electrolysis of aqueous solutions of alkali metalhalides, particularly sodium chloride, both in diaphragm cells andmercury cells where they catalyze the discharge of chloride ions whichis brought about at a re markably low and practically constantoverpotential throughout the whole life of the electrode. Under theconditions ruling in these cells, wear of the coating is insignificant,and this assures a practically unlimited life for the anodes and avoidsthe burdensome operations of opening the cells and renewing thecoatings.

SUMMARY OF THE INVENTION An object of the present invention, therefore,is to provide an electrocatalytically active coating which isparticularly well adherent to its metallic support and is resistant toelectrochemical corrosion.

This as well as other objects which will be apparent are achieved by anew type of coating for metal electrodes, which coating comprises acompound ABO, having a rutile-type structure, where A is an element inthe trivalent state selected from the group rhodium, aluminum, gallium,lanthanum and the rare earths, while B is an element in the pentavalentstate selected from the group antimony, niobium and tantalum, thecompound ABO, being associated with an oxide of the type M where M isruthenium and/or iridium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The compounds A80 and M0 mustbe present together in the coating. It may be possible to maintain theABO /MO mol ratio in a wide range for severalelectrochemical processes.7

However, it hasben found advantageous to maintain this ratio betweenwell defined limitswhen the conductive coating is intended for themanufacture of metallic anodes for the electrolysis of aqueous solutionsof alkali metal halides, especially sodium chloride. If the. proportionof M0 is too low, the overvolt age is higher than 300 mV (measuredaccording to test of Example 1) and if the proportion of M0 is too high,the consumption of noble metal/ton of produced chlorine is prohibitive,i.e., higher than 40 mg/t C1 For these reasons, the ABO /MO mol ratio ismaintained between l/l and U6.

Among all the elements for A in the trivalent state, Rh and Al havealways given better results, so they are preferred.

Further illustrative of the present invention are the followingExamples:

- Example 1 ABO .2MO where A Rh, B Sb and M v A solution A containing0.5 g-atom ruthenium/liter was prepared by dissolving rutheniumtrichloride (RuCl .xI-I O) in n-hexanol, a solution B containing 0.5g-atom rhodium/liter was prepared by dissolving rhodium trichloride(RhCl .xI-l O) in 'n-hexanol and a solution C containing 1 g-atomantimony/liter was prepared by dissolving antimony pentachloride (SbClin nhexanol.

By mixing 6.6 ml of solution A, 3.3 ml of solution B, 1.6 ml of solutionC and 38.5 ml of n-hexanol, there was obtained a composition which wasapplied in 7 layers onto plates of titanium that had previously beenhot-degreased in trichloroethylene and etched for. 5 hours atapproximately 90C in an aqueous solution of oxalic acid containing 100g/liter. For application of this composition, the titanium plates wereplaced in air on a hotplate at a temperature of about 75C.

After each application the plates were dried and then heated for 15minutes at 500C and, after the 7th application, heated for 1 hour at500C, in the presence of air.

The amount of material thus deposited was approximately 4.5 g/m.

The coating, which contained 2 g-atom Ru to l gatom Rh and l g-atom Sb,showed excellent adherence to the substrate, as was shown in strippingtests with adhesive tape applied by pressure.

The plates of titanium thus coated were submitted to two types ofevaluation.

The first relates to the over-potential for liberation of chlorine. Itis measured at an anodic current density of 10 kA/m The second relatesto the consumption of noble metal as a function of the amount ofchlorine liberated.

The measurement of over-potential is carried out by immersing the platesin brine containing 250 g NaCl/kg, saturated with chlorine at C and at apH of approximately 2. Under these conditions, the plates referred to inthis example show an initial overpotential of approximately 165 mV andthen an overpotential of 210 mV and 248 mV after a production of 67 and210 tons of chlorine/m respectively, while positively polarized under acurrent density of 10 kA/m In another test the consumption of noblemetal from these plates was evaluated by using them as anodes in a cellwith a flowing mercury cathode containing brine saturated with chlorineand sodium chloride at a temperature between and C with a constantanodecathode potential. Under these conditions the plates tested havealready produced more than 264 tons of chlorine/m at this time the limitof the useful life of the electrode had not yet been reached and theconsumption test was continued, but it was already apparent that theconsumption of noble metal would be less than 10 mg/ton of chlorineproduced.

It was 'noted that the same plates showed an overpotential ofapproximately 210 mV at 10 kA/m after producing approximately 67 tons ofchlorine/m Example 2: ABO.2MO where A Rh, B Nb and M Ru To 6 ml ofsolution A of Example 1 were added 3 ml of solution B of Example 1, 1.5ml ofa solution D containing l g-atom niobium/liter obtained bydissolving niobium pentachloride (NbCl in n-hexanol, and 39.5 ml ofn-hexanol.

Ten layers of this composition were applied to plates of titanium etchedas in Example 1. The various heat treatments were carried out in thesame manner.

The average weight of the coating thus obtained was approximately 3.7g/m it contained 2 g-atom Ru to 1 g-atom Rh and l g-atom Nb. Theadherence to the substrate was very good. When used as anodes under theconditions specified in Example 1, the plates thus coated showedover-potentials of approximately 245 mV at an anodic current density ofkA/m During the consumption test carried out as in Example 1, the amountof chlorine already produced was 81 tons/m which corresponds to aconsumption of noble metal equal to 26 mg/ton of chlorine.

Example 3: ABO.,.2MO where A A1, B Sb and M Ru A solution E containing 1g-atom aluminum/liter was prepared by dissolving aluminum trichloride(AlCl in n-butanol.

To 10 ml of solution A of Example 1 were added 2.5 ml of solution C ofExample 1, 2.5 ml of solution E and 35.0 ml of n-hexanol.

The application of this composition, in 8 coats, to 4 plates of etchedtitanium, and the heat treatments were carried out as in Example 1. Theaverage weight of the coatings thus obtained was approximately 6 g/mthey contained 2 g-atom Ru to 1 g-atom Al and l g-atom Sb. The adherenceto the substrate was excellent.

When used as anodes under the conditions defined in Example 1, theplates thus coated showed an initial over-potential of 156 mV at ananodic current density of 10 kA/m During the consumption test carriedout as in Example l, the amount of chlorine already produced was 125tons/m which corresponds to a consumption of noble metal equal to mg/tonof chlorine.

Example 4: ABO .2MO where A Rh, B Sb and M I To 2.8 ml ofa solution Fcontaining 1 g-atom of iridium/liter obtained by dissolving chloroiridicacid (H lrCl .xH O) in n-hexanol were added 2.8 ml of solution B ofExample 1, 1.4 ml of solution C of Example I and 43.0 ml of n-hexanolnThe application of this composition, in 10 coats, to plates of etchedtitanium and the heat treatments were carried out as in Example 1. Theweight ofthe coatings thus obtained was approximately 6 glm theycontained 2 g-atomlr to l g-atom Rh and 1 g-atom Sb. The adherence tothe substrate was excellent.

When used as anodes under the conditions defined in Example 1, theplates thus coated showed initial overpotentials of approximately 200 mVat an anodic current density of 10 kA/m When submitted to theconsumption test under the same conditions as in Example 1, the plateshad produced more than 172 tons of chlorine/m At this time, the limit ofuseful life of the electrode had not yet been reached and theconsumption test was continued, but it was already apparent that theconsumption of noble metal would be less than 23 mg/ton of chlorineproduced.

Example 5: ABO .4MO where A Rh, B Sb and M lr To 1.6 ml of the solutionF of Example 4 there were added 0.8 ml of solution B of Example 1, 0.4ml of solution C of Example 1 and 47.2 ml of n-hexanol.

The application of this composition, in 10 coats, on plates of etchedtitanium and the thermal treatments were carried out as in Example 1.

The weight of the coatings thus obtained was approximately 3 g/m theycontained 4 g-atom lr to l g-atom Rh and l g-atom Sb. The adherence tothe substrate was excellent.

When used as anodes under the conditions specified in Example 1, theplates thus coated showed overpotentials of approximately 230 mV at ananodic cur rent density of 10 kA/m When submitted to the consumptiontest under the same conditions as in Example 1, the plates had producedmore than 198 tons of chlorine/m At this time, the limit of useful lifeof the electrode had not yet been reached and the consumption test wascontinued, but it was already apparent that the consumption of noblemetal would be less than 11 mg/ton of chlorine produced.

Example 6: ABO .2MO where A Rh, B Sb and M Ru lr To 5.5 ml of thesolution A of Example 1 there were added 1.4 ml of the solution F ofExample 4, 4.2 ml of the solution B of Example 1, 2.1 ml of the solutionC of Example 1 and 36.8 ml of n-hexanol.

The application of this composition, in 8 coats, to plates of etchedtitanium and the thermal treatments were carried out as in Example 1.

The weight of the coatings thus obtained was approximately 7 g/m theycontained 4 g-atom Ru to 2 g-atom Ir, 3 g-atom Rh and 3 g-atom Sb; theadherence to the substrate was excellent.

When used as anodes under the conditions specified in Example 1, theyshowed an initial over-potential of approximately mV and then at 202 mVafter the liberation of 233 tons/m of chlorine, at an anodic currentdensity of 10 kA/m At this time, the limit of useful life of theelectrode had not been reached and the consumption test was continued,but it was already apparent that the consumption of noble metal would beless than 18 mg/ton of chlorine produced.

Example 7: ABO .2MO where A Rh, B Sb and M M; Ru lr To 2.1 ml of thesolution A of Example 1 there were added 2.1 ml of the solution F ofExample 4, 3.2 ml of the solution B of Example 1, 1.6 ml of the solutionC of g-atom Ir 1 and 41.0 ml of n-hexanol.

The application of the composition, in 8 coats, to plates of etchedtitanium and the heat treatments were carried out as in Example 1.

The weight of the coatings thus obtained was approximately 6 glm theycontained 2 g-atom Ru to 4 g-atom Ir, 3 g-atom Rh and 3 g-atom Sb; theadherence to the substrate was excellent.

When used as anodes under the conditions specified in Example 1, theplates thus coated showed an initial overpotential of approximately 135mV and then of 172 mV after the liberation of 150 tons of chlorine/mAfter production of 225 tons of chlorine/m the limit of useful life ofthe electrode had not yet been reached and the consumption test wascontinued, but it was already apparent that the consumption of noblemetal would be less than 17 mg/ton of chlorine produced. Example 8: ABO.2MO where A Rh, B Sb and M Ru To 15.2 ml of the solution A of Example1, there were added 7.6 ml of the solution B of Example 1, 3.8 ml of thesolution C of Example 1 and 23.4 ml of nhexanol.

The application of the composition, in 8 coats, to plates of etchedtitanium and the heat treatments were carried out as in Example 1.

The average weight of the coatings thus obtained was approximately 12.0g/m they contained 2 g-atom Ru to 1 g-atom Rh and 1 g-atom Sb. Theadherence to the substrate was very good.

When used as anodes under the conditions specified in Example 1, theplates thus coated showed an initial overvoltage of approximately 1 12mV and then ofl60 mV after the liberation of 225 tons of chlorine/m ofanodic surface at an anodic current density of kA/m At this time, thelimit of useful life of the electrode had not yet been reached and theconsumption test was continued, but it was already apparent that theconsumption of noble metal would be less than 29 mg/ton of chlorineproduced.

Example 9: ABO .4MO where A Rh, B Ta and M Ru A solution G containing0.5 g-atom/liter was prepared by dissolving ruthenium trichloride RuClx11 0 in isopropanol, a solution H containing 0.5 g-atom/liter bydissolving rhodium trichloride RhCl xH O in isopropanol and a solution 1containing 1 g-atom Ta/liter by dissolving tantalum pentachloride TaClin methanol.

By mixing 8 ml of solution G, 2 ml of solution H, l ml of solution 1 and39 ml of isopropanol, a coating composition was obtained, which wasapplied, in 10 coats, to plates of titanium etched under the sameconditions as in Example 1. After each application, the plates weredried and heated for minutes at 500C, and after the tenth application,heated for 2 hours at 500C in the presence of air.

The average weight of the coatings thus obtained was approximately 9.9g/m they contained 4 g-atom Ru to 1 g-atom Rh and l g-atom Ta. Theadherence to the substrate was very good.

When used as anodes under the conditions specified in Example 1 theplates thus coated showed an initial overvoltage of approximately 190 mVand then of 198 mV after the liberation of 55 tons of chlorine/m ofanodic surface at an anodic current density of 10 kA/m After productionof 138 tons of chlorine/m the limit of useful life of the electrode hadnot yet been reached and the consumption test was continued, but it wasalready apparent that the consumption of noble metal would be less than41 mg/ton of chlorine produced. Example 10 ABO .4MO where A Al, B Ta and5 M Ru To 10 ml of the solution A of Example 1, there were added 1.25 mlof solution E of Example 3, 1.25 ml of the solution I of Example 9 and37.5 ml of n-hexanol. The application of the composition, in 10 coats,to plates of etched titanium and the various heat treatments werecarried out as in Example 1.

The average weight of the coatings thus obtained was approximately 6.8g/m they contained 4 g-atom Ru to l 'g-atom Al and l g-atom Ta. Theadherence to the substrate was very good.

When used as anodes under the conditions specified in Example 1 theplates thus coated showed an initial overvoltage of approximately 145mV, at a current density of 10 kA/m When submitted to the consumptiontest under the same conditions as in Example 1, the plates had producedmore than 131 tons of chlorine/m of anodic surface. At this time, thelimit of useful life of the electrode had not yet been reached and theconsumption test was continued, but it was already apparent that theconsumption of noble metal would be less than 26 mg/ton of chlorineproduced.

Example 11 :ABO .2MO where A 0.9 Rh 0.] Yb, B Sb and M Ru A solution .1containing 1 g-atom Sb/liter was prepared by dissolving antimonypentachloride in isopropanol and a solution K containing 0.1 g-atomYb/liter by attacking ytterbium oxide Yb O with hydrochloric acid anddissolving in isopropanol.

By mixing 10 ml of solution G of Example 9, 4.5 ml of solution H ofExample 9, 2.5 ml of solution K, 2 ml of solution J and 30.5 mlofisopropanol, a coating composition was obtained, which was applied, in10 coats, to plates of titanium etched under the same conditions as inExample 1. The various heat treatments were carried out by the samemanner.

The average weight of the coatings thus obtained was approximately 8.0g/m they contained 2 g-atom Ru to 0.9 g-atom Rh, 0.1 g-atom Yb and 1g-atom Sb. The adherence to the substrate was excellent.

When used as anodes under the conditions specified in Example 1 theplates thus coated showed an initial overvoltage of approximately 122 mVat an anodic current density of 10 kA/m When submitted to theconsumption test under the same conditions as in Example 1, the plateshad produced more than 103 tons of chlorine/m of anodic surface. At thistime, the limit of useful life of the electrode had not yet been reachedand the consumption test was continued, but it was already apparent thatthe consumption of noble metal would be less than 41 m-g/ton of chlorineproduced.

The data of the illustrative Examples 1 to 11 are collected together inTable 1.

It will be understood that the above description of the presentinvention is susceptible to various modifications, changes andadaptations, and the same are intended to be comprehended within themeaning and range of equivalents of the appended claims.

TABLE 1.

COMPILATION OF RESULTS ACHIEVED IN THE EXAMPLES. Example Coatingcomposition Wt. of Adherence of Initial ovcr' Consumption test of thecoating No. coating the coating voltage in m\/ Cl produced noble metalcon g/m at kA/m in t/m sumcd mg/t Cl produced l RhSbO..2RuO 4.5excellent 165 264 10 2 Rl1NbO .2RuO 3.7 very good 245 8| 26 3 AlSbO.2RuO 6 excellent I56 125 4 RhSb()..2lrO 6 excellent 200 172 23 5 RhSbOl46 .4lr0 3 excellent 230 198 11 6 RhSbO .2(Ru,,;,1 )O 7 excellent 135233 18 7 RhSbO .2(Ru ,;,l )0 6 excellent 135 225 17 8 RhSbO HZRuO 12very good 112 225 29 9 RhTuO .4RuO- 9.) very good 190 138 4] l0 AlTaO.4RuO, 68 very good 145 131 26 ll (Rh Yb )SbO .2RuO 8 very good 122 10341 We claim: 2. A metallic electrode as claimed in claim 1, where l. Ametallic electrode for electrochemical processes A i an l t in thtrivalent state selected from the comprising a metal support and on atleast a portion of group consisting f Rh d All said support a conductivecoating consisting essentially 20 of a compound ABO; with a structure ofthe rutile- 3. A metallic electrode 88 claimed in claim 1, where type,where A represents an element in the trivalent B iS pentavalen Sbstateselected from the group consisting of Rh, Al, and Ga and B represents anelement in the pentavalent state selected from the group consisting ofSb, Nb and Ta, and an oxide of the type M02 where M is Selected from 5.A metallic electrode as claimed in claim 1 wherein the group consistingof Ru and Ir, and the mole ratio of the compound ABO4 is RhSbO.

4. A metallic electrode as claimed in claim 1 wherein the metal supportis a valve metal.

1. A METALLIC ELECTRODE FOR ELECTROCHEMICAL PROCESSES COMPRISING A METALSUPPORT AND ON AT LEAST A PORTION OF SAID CUPPORT A CONDUCTIVE COATINGCONSISTING ESSENTIALLY OF A COMPOUND ABO4 WITH A STRUCTURE OF THERUTILE-TYPE, WHRE A REPRESENTS AN ELEMENT IN THE TRIVALENT STATESELECTED FROM THE GROUP CONSISTING OF RH, AL AND GA AND B REPRESENTSS ANELEMENT IN THE PENTAVALENT STATE SELECTED FROM THE GROUP CONSISTING OFSB, NB AND TA, AND AN OXIDE OF THE TYPE MO2 WHERE M IS SELECTED FROM THEGROUP CONSISTING OF RU AND IR, AND THE MOLE RATIO OF ABO4 TO MO2 IS FROM1/1 TO 1/6.
 2. A metallic electrode as claimed in claim 1, where A is anelement in the trivalent state selected from the group consisting of Rhand Al.
 3. A metallic electrode as claimed in claim 1, where B ispentavalent Sb.
 4. A metallic electrode as claimed in claim 1 whereinthe metal support is a valve metal.
 5. A metallic electrode as claimedin claim 1 wherein the compound ABO4 is RhSbO4.